Light guide illumination systems with enhanced light coupling

ABSTRACT

A face-lit waveguide illumination system employing a planar slab or plate of an optically transmissive material. A light source is optically coupled a linear optical element which is formed in or attached to a face of the waveguide and is configured to inject light at an angle permitting for light propagation by means of a total internal reflection. Light is propagated through the waveguide towards a predetermined direction in response to optical transmission and total internal reflection. Light extraction features located along the prevailing path of light propagation extract light from the waveguide and emit such light towards a surface normal direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/766,698, filed Feb. 13, 2013, incorporated herein by reference in itsentirety. This application also claims priority from U.S. provisionalapplication Ser. No. 61/598,854 filed on Feb. 14, 2012, incorporatedherein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. § 1.14.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to planar light emitting waveguides suchplanar plate or slab waveguides distributing light along the broadsurface of the plate and emitting the distributed light from thebroad-area plate surface. This invention also relates to an apparatusand method of inputting light into a planar waveguide through its faceas opposed to edge-lit light guide panels where light is input throughone of the waveguide edges. More particularly, this invention relates topanel luminaires, illuminated panel signs, illuminated window panesigns, front lights, backlights, lighting panels, LCD displaybacklights, computer screens, advertising displays, road signs, and thelike, as well as to a method for redistributing light from a variety oflight sources.

2. Description of Background Art

Conventionally, light emitting devices employing a planar waveguideinclude an optically transmissive plate, a light source coupled to theplate's edge and a series of optical features distributed along a majorsurface of the plate for extracting light at predetermined locations ofthe surface.

However, a number of applications exist where edges of the waveguide arenot accessible or it is otherwise impractical to input light through anedge. Furthermore, many existing structural or artistic articles whichcan provide light guiding and distribution functions are not alwaysreadily transformable to edge-lit applications. Typical examples includeframed glass windows of building facades and doors, storefront windowpanes, as well as various interior and exterior architectural featuresemploying transparent glass or plastic panels.

Other examples of common objects which could be used as planarwaveguides but may not be suitable for light input from an edge includebut are not limited to planar slabs of glass or transparent plasticwhich edges are roughened or sanded. In a yet further example, the edgesof some transparent slabs or panels may be tapered making it difficultto input light from a relatively large source. The light input apertureof edges may also be too small compared to the size of the light sourcedue to the insufficient thickness of the transparent slab or panel.

It is therefore an object of this invention to provide an improvedillumination system providing an efficient light input through a face ofa planar waveguide as opposed to light input through an edge. It isanother object of this invention to provide a convenient light injectioninto a planar waveguide, such as an existing window pane of a building,through its face, without having to penetrate into the waveguide'ssurface. It is yet another object of this invention to provideconvenient light input in one area of a major surface of a planarwaveguide and light extraction from the waveguide in another area of thesurface. It is yet another object of this invention to provide animproved method of coupling light to a planar waveguide without havingto access its edges and while substantially reducing or eliminating theunwanted light spillage due to the coupling. Other objects andadvantages of this invention will be apparent to those skilled in theart from the following disclosure.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to face-lit planarwaveguide illumination systems which may be employed to redistributelight emitted by a compact light source over a large area of the planarwaveguide and re-emit at least a portion of the distributed light from amajor surface of the waveguide. More particularly, this invention isdirected to a system for injecting light into the face of a planarplate, slab or substrate of an optically transmissive material in onearea and extracting at least some of the injected light from anotherarea of the plate or substrate.

The present invention solves a number of problems associated with lightdistribution and illumination using planar waveguides by providing aface-lit solution which is not hindered by the limitations ofconventional edge-lit illumination devices requiring the access towaveguide edges or surface penetration for enabling light input.

An advantage of the present system is to provide controlled light inputtrough a face of the planar waveguide in one location so that such lightcan be propagated along the waveguide in response to the opticaltransmission and a total internal reflection generally towards at leastone well-defined direction and can then be extracted from anotherlocation of the waveguide along said direction.

Light is injected into the planar waveguide by means of an elongated(linear) optical element attached to a face of the waveguide andoptically coupled to said face.

According to one aspect of the invention, light is input into a face orbroad-are surface the planar waveguide non-invasively and withoutpenetrating into the waveguide's body. According to another aspect ofthe invention, a linear configuration of the optical element allows forcoupling light into the waveguide from a linear light source which maybe represented by an elongated light-emitting element or a linear arrayof compact light-emitting elements. According to yet another aspect, thelinear configuration of the light coupling optical element may also beused for injecting light into the waveguide from a discrete light sourceoptically coupled to a terminal end of the optical element.

In at least one embodiment, the invention features a planar opticalwaveguide exemplified by a portion of an optically transmissive windowpane, a linear light coupling optical element attached to a planar faceor major broad-area surface of the pane and a linear light source. Thelight source includes a strip light emitting diodes (LEDs) incorporatedis a linear array. The linear LED array is positioned generally parallelto the longitudinal axis of linear optical element and is configured toilluminate the optical element with a beam of light. The linear opticalelement is optically coupled to the face of the window pane by means ofa planar surface and by providing a good optical contact between thesurface and the face of the window pane.

The optical element is configured to inject light into the medium of thewindow pane at a sufficiently low out-of-plane angle permitting for thesubsequent light propagation within by means of a total internalreflection from the opposing faces of the pane. According to an aspectof the present invention, the optical element injects light into thewaveguide mode while directing the light beam towards a well-defineddirection along the window pane so that the injected light can be mixedalong the propagation path and then extracted at another location of thewindow pane.

In at least one embodiment, the invention includes one or more lightextracting features positioned along the prevailing path of lightpropagation in the waveguide. The light extracting features areconfigured to extract light from the waveguide and direct such lighttowards a surface perpendicular. According to an aspect of theinvention, at least a portion of the extracted light may be directedaway from the waveguide and towards a viewer thus providing conspicuousvisibility of the light extracting features.

In at least one embodiment, the invention includes a linear collimatingelement optically coupled to the light source. Various implementationsof the collimating element include linear or axisymmetrical refractivelenses, TIR lenses, reflectors, concentrators, and any combinationsthereof.

Various implementation of the light coupling optical element includeforming at least a part of the element from an elongated block oftransparent material. In one implementation, the elongated block has ashape of a right-angle prism or wedge. In one implementation, theelongated block has at least one planar surface or at least onecurvilinear surface. In one implementation, the elongated block has atriangular transversal cross-section. In one implementation, theelongated block has a trapezoidal transversal cross-section. In furtherimplementations, the optical element has sharply asymmetrical andaxisymmetrical configurations in a transversal cross-section. Variousfaces or surfaces of the optical element may be configured forrefracting light and/or for reflecting light by means of a specularreflection or TIR.

In at least one embodiment the light coupling optical element includes astrip of a film material laminated onto the face of the waveguide.Various implementations of such optical element include prismatic films,holographic films, diffractive films, and any other types of lightturning films which may be used for suppressing the refraction at thewaveguide surface or for injecting light into waveguide at anglesallowing for TIR propagation.

Various implementations of the face-lit waveguide illumination systeminclude suitable housing components and means for blocking stray light.

In at least one embodiment of the invention, the width of the linearoptical element is defined by the thickness of the planar waveguide.According one specific implementation, the width of the optical contactarea of the linear optical element with a major surface of the waveguideis approximately equal to or less than 2d/√{square root over (n²−1)},where d is the thickness of the waveguide and n is the refractive indexof the waveguide's medium. According to other specific implementations,the transversal width of the light coupling optical element is less thantwo times the thickness of the planar waveguide and more preferably, 1.8times the thickness of a glass window pane.

In at least one embodiment, the light source is selected from the groupof light emitting elements including fluorescent lamps, linear arrays oflight emitting diodes, incandescent lamps, cold-cathode or compactfluorescent lamps, halogen, mercury-vapor, sodium-vapor, metal halide,electroluminescent lamps or sources, field emission devices, and lasers.

In at least one embodiment, the face-lit waveguide illumination systemincludes the light coupling optical element made from light transmittingmaterial and having the shape of a linear rod or bar. Such rod or barmay have various cross-sections including but not limited to square,rectangular, triangular, pentagonal, hexagonal, octagonal, trapezoidal,circular, half-circular, and oval. The cross-section may also includecircular segments or sectors.

In at least one embodiment, the optical element includes an opticallytransmissive light turning or light redirecting film. Variousimplementations of such film include microstructured prismatic films,diffractive films, holographic films, and films with internal lightredirecting structures.

In at least one embodiment, the illumination system includes means forextracting light from the planar waveguide. In one implementation, thelight extracting means include light scattering or light diffusingfeatures. Such features may be embedded into the waveguide's body formedin a surface of the waveguide or externally attached to the waveguidesurface. In other implementations, the light extracting means include atextured surface, indicia and or an image print.

In at least one embodiment, the light coupling optical element may beconfigured with wave guiding properties. In different implementations,it may be formed by a relatively long elongated body of a lighttransmitting material or include one or more optical fibers.

In at least one embodiment, the light coupling optical element isattached to the face of the waveguide by means of an intermediate filmwhich may have a substantially larger area than the contact area of theoptical element. In at least one embodiment, the optical element isattached either directly to the face of the waveguide or to theintermediate film or plate using an optically clear adhesive orencapsulant.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic perspective view of a face-lit waveguideillumination system, according to at least one embodiment of the presentinvention.

FIG. 2 is a schematic cross-sectional view and raytracing of a face-litwaveguide illumination system portion, showing a wedge-shaped lightcoupling optical element, according to at least one embodiment of thepresent invention.

FIG. 3 is a schematic cross-sectional view and raytracing of a face-litwaveguide illumination system portion, showing a tapered light couplingoptical element, according to at least one embodiment of the presentinvention.

FIG. 4 is a schematic cross-sectional view and raytracing of a face-litwaveguide illumination system portion, showing a symmetricalconfiguration of a light coupling optical element, according to at leastone embodiment of the present invention.

FIG. 5 is a schematic cross-sectional view and raytracing of a face-litwaveguide illumination system portion, showing a prismatic filmlaminated onto a face of a window pane, according to at least oneembodiment of the present invention.

FIG. 6 is a schematic view of a face-lit waveguide illumination systemportion, showing an exemplary arrangement of linear light emitting andlight coupling elements, according to at least one embodiment of thepresent invention.

FIG. 7 is a schematic view of a face-lit waveguide illumination systemportion, showing a layered light coupling element including internalcorrugated boundary between two layers, according to at least oneembodiment of the present invention.

FIG. 8 is a schematic view of a face-lit waveguide illumination systemportion, showing an optical element having curvilinear reflectivesurfaces, according to at least one embodiment of the present invention.

FIG. 9 is a schematic cross-sectional view and raytracing of a face-litwaveguide illumination system, showing a reflective surface extendingparallel to a face of a window pane, according to at least oneembodiment of the present invention.

FIG. 10 is a schematic cross-sectional view and raytracing of a face-litwaveguide illumination system, showing a light source optically coupledto an edge of a planar light coupling optical element, according to atleast one embodiment of the present invention.

FIG. 11 is a schematic perspective view and raytracing of a face-litwaveguide illumination system, showing a planar light coupling opticalelement attached to a face of a window pane, according to at least oneembodiment of the present invention.

FIG. 12 is a schematic perspective view and raytracing of a face-litwaveguide illumination system portion, showing a plurality of opticallytransmissive rods or bars attached to a face of a planar slab waveguide,according to at least one embodiment of the present invention.

FIG. 13 is a schematic cross-sectional view of a face-lit waveguideillumination system portion, illustrating light injection into a thinplanar waveguide, according to at least one embodiment of the presentinvention.

FIG. 14 is a schematic view of a face-lit waveguide illumination systemportion, illustrating an exemplary method of attaching a light couplingoptical element to a face of a planar waveguide, according to at leastone embodiment of the present invention.

FIG. 15A through FIG. 15F, illustrate various exemplary configurationsof a linear optical element, according to at least some embodiments ofthe present invention.

FIG. 16 illustrate an exemplary arrangement of optical elements andlight sources with respect to a face of a window pane, according to atleast one embodiment of the present invention.

FIG. 17 is a schematic perspective view and raytracing of a face-litwaveguide illumination system portion, showing a plurality of opticalfibers attached to a face of a planar slab waveguide, according to atleast one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the system generally shown in thepreceding figures. It will be appreciated that the system may vary as toconfiguration and as to details of the parts without departing from thebasic concepts as disclosed herein. Furthermore, elements represented inone embodiment as taught herein are applicable without limitation toother embodiments taught herein, and in combination with thoseembodiments and what is known in the art.

A wide range of applications exist for the present invention in relationto the collection and distribution of electromagnetic radiant energy,such as light, in a broad spectrum or any suitable spectral bands ordomains. Therefore, for the sake of simplicity of expression, withoutlimiting generality of this invention, the term “light” will be usedherein although the general terms “electromagnetic energy”,“electromagnetic radiation”, “radiant energy” or exemplary terms like“visible light”, “infrared light”, or “ultraviolet light” would also beappropriate.

Furthermore, many applications exist for the present invention inrelation to distributing light by means of a planar optical waveguidewhich hereinafter may also be referenced to as a light guide. The planaroptical waveguide refers to a broad class of objects employing anoptically transmissive material confined between two opposing broadsurfaces which are substantially parallel to each other. The termsubstantially parallel generally includes cases when the opposingsurfaces are parallel within a predetermined accuracy, particularlyincluding the cases when the body of the material defined by the abovesurfaces has a slightly tapered shape or has a variable thickness acrossthe surface.

According to the present invention, the planar waveguide may beexemplified by a transparent plate, slab, panel, pane,light-transmitting substrate or any suitable sheetform of an opticallytransmissive material. This invention is also applicable to anytwo-dimensional shape variations of the sheetforms, including but notlimited to a rectangle, a polygon, a circle, a strip, a freeform, or anycombination therein. This invention is further applicable to anythree-dimensional shapes that can be obtained by bending the sheetformsaccordingly, including but not limited to cylindrical orsemi-cylindrical shapes, conical shapes, corrugated shapes, and thelike.

The present invention seeks to provide illumination systems capable ofcoupling light into a planar waveguide through its broad-area front orback surface in one location and extracting the coupled light at adifferent location of the waveguide.

According to the present invention, there is provided an illuminationsystem employing a waveguide exemplified by an optically transmissivepanel which may also hereinafter be referred to as a light guide panelor LGP. The panel is made from a material which has a refractive indexgreater than that of the outside medium and is therefore inherentlycapable of guiding light within the panel by means of a Total InternalReflection (TIR) from its opposing broad surfaces, provided that theinternal incidence angles onto either of the surfaces are greater than acritical angle of TIR characterizing the surfaces.

For the purpose of this discussion, the term “incidence angle” of alight ray in relation to a surface generally refers to an angle thatthis ray makes with respect to a normal to that surface. It will beappreciated by those skilled in the art of optics that, when referringto light or other waves passing through a boundary formed between twodifferent refractive media, such as air and glass, for example, theratio of the sines of the angles of incidence and of refraction is aconstant that depends on the ratio of refractive indices of the media(the Snell's law of refraction). The following relationship can describea light bending property of an interface between two refractive media:n_(I) sin ϕ_(I)=n_(R) sin ϕ_(R), where n_(I) and n_(R) are therespective refractive indices of the materials forming the opticalinterface and ϕ_(I) and ϕ_(R) are the angle of incidence and the angleof refraction, respectively. It will be further appreciated that suchoptical interface can also be characterized by a critical TIR anglewhich is the value of ϕ_(I) for which ϕ_(R) equals 90°. Accordingly, fora surface characterized by a stepped drop in refractive index along thepropagation path of a ray, the incidence angle may be less than, equalto, or greater than the TIR angle at the given surface.

A TIR angle ϕ_(TIR) can be found from the following expression:ϕ_(TIR)=arcsin(n _(R) /n _(I)·sin 90°)=arcsin(n _(R) /n _(I))  (Equation1)

In an exemplary case of the interface between glass with the reflectiveindex n_(I) of about 1.51 and air with n_(R) of about 1, ϕ_(TIR) isapproximately equal to 41.5°.

It will be appreciated that, once light is input into the planar lightguide, such as LGP, and its propagation angles permit for TIR to occurat LGP's longitudinal walls, the light becomes trapped in LGP and canpropagate considerable distances until it is extracted, absorbed orreached an edge of the panel.

The present invention will now be described by way of example withreference to the accompanying drawings.

FIG. 1 depicts an embodiment of a face-lit waveguide illumination system2 in accordance with the invention. System 2 includes a planar lightguide exemplified by a rectangular glass window pane 4 which is mountedwithin a rigid frame 300. Window pane 4 thus has two opposing parallelbroad-area surfaces which are exposed to the air and four edges whichare conventionally covered by frame 300.

While there are many uses for the face-lit illumination system of thisinvention, the embodiments described here are directed particularly tousing various optically transparent panels or panes which are parts ofthe existing exterior or interior structures and/or decorative elementsof buildings. Useful examples of such panels include but are not limitedto framed or unframed vertical wall windows, door windows, glass ortransparent-plastic facades of buildings, glazed openings in walls andceilings, vertical or horizontal interior space partitions, screens, andthe like. It will be appreciated that such types of panels, albeit oftenproviding a good optical transparency, are not commonly designed forillumination purposes. Yet, many of them represent convenient existingstructures which are highly transparent and shaped in the form of aplate having smooth broad-area surfaces, which can make them usable forillumination purposes or for providing various lighting effects to anobserver. It is therefore an object of this invention to provide meansfor utilizing such already existing optically-transmissive structures asplanar optical waveguides transporting light from one location toanother so that light can be injected into the face of the respectivewaveguide, distributing such light through the waveguide's body and emitthe injected light from another location of the waveguide without havingto access its edges for light input.

Accordingly, by way of example and not limitation, window pane 4 may beof the type typically found in single-pane or dual-pane windows or doorsof the residential or commercial buildings. In another non-limitingexample, window pane 4 may be a part of a storefront window in ashopping mall. It will be appreciated that such windows or doors,although being typically transparent and optically clear, generally haveat least some or all of the edges of the window pane covered with aframe and are therefore not suitable for conventional, edge-litapplications.

System 2 of FIG. 1 further includes a light coupling optical element 6made from an optically transmissive material and configured forinjecting light into pane 4 through its face as opposed to injectinglight through an edge in edge-lit illumination systems. Optical element6 is attached to one of the two opposing broad-area surfaces or faces ofwindow pane 4 and is optically coupled to the pane with refractive indexmatching. According to the preferred to embodiments, it is importantthat optical element 6 is attached to the face of window pane 4externally, that is without surface penetration.

The term “optically coupled” is directed to mean any relationshipbetween two optical components which enables light to pass from oneoptical component to the other without being redirected back orabsorbed. The term “refractive index matching”, in relationship to anoptical coupling, is directed to mean such optical coupling between tworefractive optical components where the refractive indices of bothcomponents are selected to approximate each other.

In the context of the present invention, index matching may serve acongruence of goals. One particular goal of the index matched opticalcoupling is reducing or eliminating the Fresnel reflections at therespective optical interface in a broad range of incidence angles.Another goal may be eliminating or suppressing TIR in an exemplary casewhere the refractive index of a first optical element is less than thatof the second optical element and where the light is designed topropagate from the first element to the second element. In this case,the proper index matching should ensure that no TIR will occur at therespective optical interface within the designed range of incidenceangles and that essentially all of the light will be transmitted to thesecond optical element. It should be understood that index matchinggenerally precludes forming any air gap between the two refractiveoptical elements.

For the purpose of this invention and from the practical standpoint, twooptical elements may also be considered optically coupled with indexmatching when they are disposed in a tight optical contact, directly orthrough one or more intermediate optical layers, and when thelight-receiving optical element has a greater refractive index than thelight-emitting component or, if lower, when the differences between therefractive indices are less than a predefined sufficiently small amount.

According to one embodiment, optical element 6 may be glued to thesurface of pane 4 using an optically clear adhesive which refractiveindex is greater or approximately equal to the refractive index of thematerial of element 6. The adhesive may have a relatively high tack forpermanent bond or a relatively low tack permitting for the eventualremoval of element 6 from pane 4. According to one embodiment, opticalelement 6 may be coupled to pane 4 using an intermediate optically clearsubstrate which has a similar refractive index and which, in turn, isattached to the surface of pane 5 with a good optical contact.

Referring further to FIG. 1, optical element 6 has an elongated shapewith a longer dimension being substantially greater than the shorterdimensions and is exemplified by a linear prism having a triangulartransversal cross-section. Suitable materials for making such linearprism include but are not limited to water-clear glass, Poly(methylmethacrylate) (PMMA), polycarbonate, styrene, cured urethane, silicone,and the like. Optical element 6 is positioned near a bottom edge ofwindow pane 4 with its longitudinal axis 44 aligned generally parallelto the bottom edge of the pane and perpendicular to an intendedprevailing direction 98 of light propagation. In the case illustrated inFIG. 1, direction 98 generally points from the bottom edge of pane 4towards its top edge.

It is noted that the illustrated position and orientation of opticalelement 6 with respect to the edges of pane 4 should not be constructedas limiting this invention in any way. Element 6 may also be positionedat any other location of the optically transmissive surface of pane 4.Particularly, element 6 may be located at and aligned with respect toany other edge of pane 4, including the vertical edges and the top edge.In one embodiment, optical element 6 may also be located at aconsiderable distance from all of the edges, including the case when itis located near the center of pane 4. Additionally, linear opticalelement 6 may also be disposed in any other suitable orientation of itslongitudinal axis 44 with respect to the edges of pane 4, includingperpendicular and angular positions.

Referring yet further to FIG. 1, system 2 additionally includes a lightsource 30 exemplified by a linear array or a strip of light emittingdiodes 32 illuminating at least one longitudinal face of the prism thatforms optical element 6. Optical element 6 is designed to intercept atleast a substantial portion of light emitted by source 30 andcommunicate such light into pane 4 so that light source 30 becomesoptically coupled to pane 4 by means of element 6.

It should be understood that light source 30 is not limited to lightemitting diodes (LEDs) and may include any suitable single or multiplelight sources of any known type, including but not limited to:fluorescent lamps, incandescent lamps, cold-cathode or compactfluorescent lamps, halogen, mercury-vapor, sodium-vapor, metal halide,electroluminescent lamps or sources, field emission devices, lasers,etc. In a linear configuration, light source 30 may comprise a singlelinear light-emitting element or may include two or more compact lightemitting elements incorporated into a linear array. When light source 30includes multiple light emitting elements, each of such elements mayhave any suitable shape, including compact or extended two-dimensionalor one-dimensional (elongated) shapes.

Light source 30 may further include integrated or external optics suchas collimating or light-redistributing lenses, mirrors, lens arrays,mirror arrays, light diffusers, waveguides, optical fibers and the like.When multiple light emitting elements are employed, each of suchelements may be provided with individual optics. Alternatively, a singlelinear optic may be provided for the entire array to collimate light orotherwise shape the emitted beam in a plane which is perpendicular tothe longitudinal axis of the array.

Referring yet further to FIG. 1, an optional housing 302 may be providedto cover or encase light source 30 and/or optical element 6. By way ofnon-limiting example, housing 302 may have different functions includingbut not limited to structural, protective (from dust, moisture,elements, impact, etc.) and/or aesthetic.

Light source 30 may be provided or associated with an electric powersource exemplified by an AC/DC power supply 306 connected to source 30and an AC power grid using an electric cord 304. The power source maycomprise any conventional power sources used to power various lightsources. The power sources may include, for example, one or moreelectric power supplies, batteries, transformers, LED drivers, variousother types of voltage and/or current converters or power conditioningunits.

System 2 further includes a light emitting component attached to a faceof pane 4 and including one or more light extracting features 20. Lightextracting features 20 may include any light redirecting structure ordevice configured for extracting light from pane 4.

In an embodiment illustrated in FIG. 1, the light emitting component isexemplified by a rectangular piece of optically transmissive film 8applied to a surface of pane 4. Light extracting features 20 areexemplified by light-scattering areas formed in or attached to film 8.Each of the light-scattering areas has a two-dimensional shape oroutline representing an individual letter so that the series oflight-scattering areas represent the word “OPEN”.

It should be understood, however, that the light emitting areas are notlimited to letters, words or characters and may have any othertwo-dimensional shapes which, in turn, may be arranged in any suitablemanner. By way of non-limiting examples, features 20 may also representany geometric shapes, symbols, indicia, images or patterns.Alternatively, features 20 may represent no particular shape or patternand may simply provide a uniform or non-uniform glow for illumination orvisual effect function.

Film 8 may be applied to the same face of pane 4 as the optical element6 or to the opposing face. Film 8 should be applied to the respectiveface with a good optical contact which should be sufficient to enablethe interaction of light that may propagate in pane 4 with features 20.

The attachment of film 8 to the face of pane 4 may involve the processof lamination with or without adhesive. When no adhesive is used, thematerial of film 8 should preferably have static cling properties andthe contacting surface of film 8 should preferably be calendered toprevent unwanted delamination. In an alternative, film 8 may belaminated onto pane 4 using an optically transmissive adhesive layer.Suitable materials for film 8 include but are not limited to clear ortranslucent vinyl, acetate, PET, and polyethylene. Many common types ofdecal films used for signage may also be suitable for film 8.

Light extracting features 20 may include any suitable means forreceiving and redistributing light and should generally be configured toextract light from pane 4. Features 20 may be configured to extractlight by means of scattering, reflection, refraction, deflection,diffraction and may be further configured to do so with changing one ormore properties of light. Exemplary properties of light that may bechanged by film 8 and/or features 20 include but are not limited towavelength, polarization, apparent brightness, spectral distribution,angular and/or spatial distribution, dispersion, etc. According to oneembodiment, film 8 is optically clear and configured to generallypreserve the spectral properties of light propagating in pane 4.According to one embodiment, film 8 may have specific color-filteringproperties thus being able to change the color of light that passesthrough the film.

By way of a more specific illustrative example of light-extracting film8, features 20 may include light-diffusing surface microstructuresformed in the surface of the film. The microstructures may includeordered or random surface relief features formed, for example, by meansof etching, embossing, laser ablation, sanding, micromachining,micro-replication and any other method suitable for producing thedesired surface texture or relief. In a further non-limiting example,features 20 may be formed by depositing a layer of light scatteringmaterial, such as ink or paint onto the surface of film 8 or directlyonto the surface of pane 4.

In one embodiment, light extracting features 20 may have phosphorescentor fluorescent properties. For example, features 20 may include one ormore shapes cut from a sheet of fluorescent material that convertsshorter wavelengths of light in the UV or visible spectrum into longerwavelengths in the visible range. Such sheet-form shapes may belaminated onto the surface of pane 4 and configured to scatter lightwith fluorescent effect when illuminated with a light source. By way ofexample and not limitation, the fluorescent material may be configuredto convert 350 nm-400 nm UV light from a “black light” into visiblewavelengths.

In one embodiment, light extracting features may include an image printwhich is made on a sheet of paper or printable polymeric material. Suchprint should preferably be made using inks or paints that becomeparticularly conspicuous when illuminated by a light source.

Film 8 and particularly features 20 may be located at any suitable areaof pane 4 surface. In at least some embodiments, it may be preferredthat features 20 are disposed in a generally different location of thesurface of pane 4 than optical element 6 and light source 30. In oneembodiment, features 20 may be located in a mid-portion of pane 4 whilelight source 30 and/or optical element 6 are located at an edge andmasked from viewing. In such an arrangement, a distinct visualappearance of glowing features 20 in the middle of the highlytransparent glass window pane may be realized with a minimum glare fromlight source 30, thus providing a more conspicuous optical effect.

The mutual disposition of light coupling optical element 6 and lightextracting features 20 should be such that at least a portion of lightemitted by source and injected into pane 4 can be intercepted by thelight extracting features. In one embodiment, It is also preferred thatoptical element 6 is designed to inject light into pane 4 whiledirecting such light generally towards the location of features 20. Moreparticularly, in order to maximize light extraction, it may be preferredthat direction 98 generally points towards the center of an area formedby light extracting features 20.

In operation, light emitting diodes 32 of source 30 emit light towardslight coupling optical element 6 which, in turn, injects said light intopane 4 generally at an acute injection angle with respect to the planeof pane 4 and also directs such light towards light extracting features20. The injection angle should be sufficiently low to result thesubsequent light propagation between the opposing faces of pane 4 in awaveguide mode. In other words, the incidence angle of light rays ontothe opposing faces of pane 4 resulting from the light injection shouldgenerally be greater than the TIR angle at each of the faces.

It will be appreciated that, when the TIR conditions are met at leastfor one of the two faces of pane 4, they will generally be met for theopposing parallel face as well. Accordingly, light injected into pane 4may propagate considerable distances along its broad-area surfaces whichmay involve a number of light bounces from the opposing faces by meansof TIR. The effective distance of light propagation along the pane isprimarily defined by the degree of optical transmissivity of the pane′material and by the surface smoothness. It will be appreciated that manycommon objects such as glass panes of windows and door, acrylic sheetsand polycarbonate sheets are generally capable to guide light todistances from at least several centimeters with negligible attenuationup to several meters with some more appreciable attenuation.

At least some types of commercially available water-clear architecturalglass used for storefronts have reduced iron content and often haveexceptional light transmission properties. Framed window panes made fromsuch types of glass may be particularly suitable for face-lit waveguideillumination systems disclosed in this invention. As light propagatesbetween the opposing broad-area surfaces of pane 4 towards features 20in response to transmission and TIR, at least a portion of thepropagating light will reach features 20. Each of features 20 scattersthe incident light so that at least a substantial portion of thescattered light is extracted from pane 4 towards the viewer. As aresult, features 20 will appear brighter than the surrounding backgroundsurfaces of film 8 and pane 4.

It will be appreciated that the apparent brightness and color offeatures 20 may be controlled by the brightness and color of source 30thus providing a convenient means of making visually appealing andconspicuous illuminated signs. It will be appreciated that an embodimentillustrated in FIG. 1 may be conveniently used for making varioussignage using existing building elements, such as windows or door glasspanels, and without undue structural intrusion. Also, it is noted thatthis invention may be applied to a variety of panel lighting and signageapplications where edge-lit LGPs have been traditionally employed. Sincethe light injection components of system 2 can be attached to eitherfront or back surface of the LGP, no access to the panel's edge isnecessary which may enhance the utility of the device, especially in theapplications where the access to the edges is difficult or unwanted.

The light injecting and light extracting components of system 2 may bemade removable thus making the device practical for temporary, low-costsignage. For example, film 8 that includes suitable light extractingfeatures can be laminated onto the respective face of pane 4 using alow-tack adhesive or using static cling. When no longer needed, suchfilm 8 may be peeled off and discarded or stored for future use.Similarly, light coupling optical element 6 may be attached to a face ofpane 4 using a low-tack adhesive or pressed against the pane surface toeliminate the air gap. An index matched, optically transmissive filleror encapsulant may be used to assist in gapless optical coupling ofelement 6 to pane 4. Suitable encapsulant materials may includesilicones, ethylene vinyl acetate (EVA) or any other soft and clearmedium. Alternatively, optical element may be permanently bonded to anoptically clear film which, in turn, may be laminated onto therespective face of pane 4 using static cling or low-tack adhesive foreasy removability.

FIG. 2 depicts, in a cross-section, a portion of optically transmissivewindow pane 4 and an exemplary configuration of the linear prism formingelement 6. Window pane 4 has parallel broad-area surfaces 10 and 12opposing each other and representing the opposing faces of the pane. Thelinear prism is formed by a wedge-shaped block of a transparent materialwhich has a sharply asymmetric configuration in a transversalcross-section with the taper of the respective wedge generally pointingtowards direction 98. Particularly, the prism has the transversalcross-section of a scalene right-angle triangle which hypotenuse isfacing surface 10 of pane 4. In the context of this invention, the term“transversal cross-section” of an elongated or linear body is directedto mean a cross-section in a plane which is perpendicular to theprevailing longitudinal axis of said body.

By way of example and not limitation, such linear prism may be made byextrusion or injection molding from bulk acrylic (PMMA) or polycarbonatematerial. In a further non-limiting example, the prism may be made bymachining of a rectangular block, plate or slab of acrylic,polycarbonate or glass with the subsequent surface polishing to a highgloss.

The linear prism which forms optical element 6 has a light input face 14represented by a shorter cathetus of the respective right-angletriangle, a smooth and planar coupling face 16 represented by thetriangle's hypotenuse, and a reflective face 18 represented by thelonger cathetus. Faces 14 and 16 and faces 16 and 18 make acute dihedralangles with respect to each other pairwise. Face 16 is attached tosurface 10 with a good physical and optical contact so that element 6becomes optically coupled to pane 4. A layer 28 of index matching clearadhesive is provided to promote the adhesion and optical contact. Therefractive index of layer 28 is preferably selected to be greater orapproximately match that of element 6 and should ensure that light canfreely pass from element 6 to pane 4 in a broad range of incidenceangles, without the risk of TIR at the respective optical interface.

LED-based light source 30 is provided in the immediate vicinity of face14 of element 6. Each of light emitting diodes 32 of light source 30 mayhave an integrated collimating lens 38 of a refractive type. Lens 38 mayhave a spherical aperture and may be designed to improve lightextraction from LED 32 and/or narrow the angular distribution of lightemitted by the respective LED chip. The light output surface of source30 is disposed adjacent to face 14 so that the emitted light can freelyenter into element 6 without substantial light spillage. It will beappreciated that, when face 14 is planar and is exposed to anon-parallel beam of incident light, said light will undergo refractionso that the angular distribution of the beam will be generally narrowerwithin element 6 than in the air.

The slope of face 14 with respect to the surface of pane 4 is selectedso that at least a substantial portion of light entering element 6 makessufficiently high angles with respect to a normal to the surface of pane4. Particularly, the resulting angles of light propagation should allowfor TIR at least from the portions of surfaces 10 and 12 exposed to thelow-n outside medium, such as air. Accordingly, light entering pane 4 atsuch sufficiently high incidence angles will be reflected at least oncefrom surface 12 by means of TIR.

It will be appreciated that, once the condition for TIR at surface 12 ismet, it will also be automatically met for the opposing surface 12 dueto the parallelism of surfaces 10 and 12. Therefore, any ray propagatingat sufficiently high incidence angle with respect to a surface willbecome trapped in pane 4 and can propagate in a waveguide mode bybouncing from surfaces 10 and 12.

Optical element 6 should preferably be designed to prevent or at leastminimize secondary interactions of light rays with face 16 and thusprevent or minimize light decoupling from pane 4 through element 6.Particularly, rays injected into pane 4 at near-TIR angles shouldgenerally strike only a free portion of surface 10 in order to preventre-entering into element 6 and escaping from the waveguide mode.

For this purpose, the transversal width of coupling face 16 (representedby the hypotenuse of the respective right-angle triangle in FIG. 2) maybe selected to not exceed a certain maximum width w_(max). The maximumwidth w_(max) may be defined from various considerations which mayaccount, for example, for the angular distribution of the beam emittedby source 30, the transversal size of source 30, the refractive indicesof element 16 and pane 4, as well as the thickness of pane 4.

In one embodiment, the maximum width w_(max) may be defined from thefollowing relationship:

$\begin{matrix}{{w_{\max} = \frac{d}{2{\tan\left( \phi_{TIR} \right)}}},} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$where d is the thickness of pane 4 and ϕ_(TIR) is the critical TIR anglecharacterizing surfaces 10 and 12 of the pane. Considering that themedium surrounding pane 4 can commonly be air (n_(R)≈1) and thatEquation 1 then translates into sin (ϕ_(TIR))=1/n_(I), obtain aftersimplification:

$\begin{matrix}{w_{\max} = {\frac{2d}{\sqrt{n_{I}^{2} - 1}}.}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

In an exemplary of a glass window pane 4, n_(I)≈1.51 and ϕ_(TIR)≈41.5°,which gives w_(max)≈1.8d. To put it differently, the ratio between thetransversal width of the contact area of linear optical element 6 withthe face of a glass window pane and the thickness of such pane shouldgenerally not exceed 1.8. At this ratio, the acceptance angle of theoptical pair formed by glass pane 4 and linear optical element 6 may bemaximized since any ray which enters into pane 4 at greater-than-TIRincidence angle will be captured into the waveguide mode. In moregeneral terms, the width of the elongated body of optical element 6should normally be less than twice the thickness of pane 4 it isattached to.

In a further, more specific example of a 6-mm window pane, thetransversal width of face 16 should generally not exceed 10-11 mm. Atthe same time, it is noted that the transversal width of input surface14 (represented by the length of the shorter cathetus of the respectivetriangle in FIG. 2) should be made sufficient to accommodate the lightemitting aperture of light source 30.

It will be appreciated from the above examples that a thicker windowpane 4 will generally accommodate a wider contact surface of linearlight-injecting optical element 6 and a larger-aperture source 30.However, it should be understood that other factors may also influencethe size selection for element 16 and/or light source 30. Particularly,when the light beam from source 30 can be collimated to a narrow angularspread, the angular distribution of light in the waveguiding plate ofpane 4 may also be made sufficiently narrow. A sufficiently narrow beam,in turn, may be coupled into pane 4 at lower out-of-plane angles thusextending the distance between the individual bounces of a light rayfrom surface 10 and generally allowing for the use of a wider opticalelement 6.

Referring further to FIG. 2, surface 18 is configured to intercept rayspropagating away from surface 16 and redirect them into pane 4. Theangular disposition of face 18 with respect to faces 14 and 16 is soselected that that at least a substantial portion of light strikingsurface 18 is reflected by means of TIR with minimum or no light exitinginto the surrounding medium. Additionally, the angle of face 18 shouldbe selected so that the reflected rays obtain sufficiently high angleswith respect to a normal to surfaces 10 and 12, generally above therespective TIR angle, and thus can also be coupled into the waveguidemode.

It will be appreciated that light coupled to pane 4 according to theabove principles will propagate along the opposing pane faces away fromlight source 30 and along direction 98 by bouncing from opposing smoothbroad surfaces 10 and 12. At least a portion of such light mayeventually reach light extracting features 20 (not shown in FIG. 2)located at a distance from source 30 along direction 98. Lightextracting features 20 may thus extract (decouple) light from the panethus providing the desired illumination function.

FIG. 3 illustrates an alternative configuration of optical element 6which comprises planar faces 14, 16, 18 and further comprises a face 22which extends generally parallel to face 16 and defines a planar portionof the optical element. The planar portion of element 6 is terminatedwith a wedge portion defined by face 18. Face 14 of FIG. 3 is disposedperpendicular to surface 10 and is configured to accept light in a broadrange of incidence angles. Light source 30 comprising a linear array oflight emitting diodes 32 is positioned adjacent to face 14. The opticalaxis of each light emitting diode 32 is aligned parallel to the plane ofpane 4. Layer 28 of index-matched optical adhesive of any other type ofcoupling agent is provided along the extent of surface 16 so that thereis a good optical contact between element 6 and pane 4.

In operation, referring to FIG. 3, light emitting diode 32 illuminatesface 14 by a divergent beam of light. Face 14 intercepts at least asubstantial portion of the emitted light and admits said light intooptical element 6 with at least some refraction which somewhat narrowsthe angular beam spread. About half of light entering element 6 iscommunicated to pane 4 directly through the optical interface defined byface 16. The rest of light enters pane 4 after first reflecting fromface 22 and/or face 18. It will be appreciated that face 22 extendingparallel to surface 10 does not change the propagation angle withrespect to a normal to surface 10. Therefore, rays initially propagatingin element 6 at greater then TIR incidence angles will maintain suchangles after bouncing from face 22. The slope of face 18 with respect tosurface 10 may be selected so that most rays striking said surface areredirected into pane 4 at angles which permit for TIR propagation and sothat light escape is minimized.

Thus, the configuration of optical coupling depicted in FIG. 3 alsoallows for injecting at least a substantial portion of light emitted bysource 30 into pane 4 through its planar face. Notably, light becomestrapped within pane 4 by means of TIR so that it can be transportedalong direction 98 and further extracted by light extracting features 20(not shown in FIG. 3) at a different location of the pane.

It will be appreciated that, besides providing light distribution andtransport to features 2, pane 4 may provide convenient means for mixingand homogenizing light emitted by the LED array. Each of the lightemitting diodes 32 may be configured to emit light in the same color inwhich case features 20 can be uniformly illuminated by that color.

The LED array may be controlled in a number of ways. In one embodiment,the LED array may be powered on and off by applying the same constant orvariable current to each LED. Alternatively, different LEDs 32 in thearray may be configured to emit different colors and each of suchdifferent-color LEDs 32 may be controlled individually. Since lightbeams from individual LEDs 32 mix in pane 4, this can create variouscolor and brightness effects as light is scattered by the medium of pane4 or extracted by light extracting features 20. LEDs 32 may also bearranged in clusters of different-color emitters, such as red-green-blue(RGB) or the like, and the color of the emission may be controlled bychanging the intensity of individual LEDs in each cluster.

FIG. 4 depicts a further alternative configuration of light couplingcomponents of system 2. Referring to FIG. 4, there is provided anoptically transmissive planar light guide represented by a portion ofwindow pane 4 having opposing broad-area surfaces 10 and 12 extendinggenerally parallel to each other. System 2 further comprises opticalelement 6 including two identical linear prisms 56 and 356 disposed in asymmetrical configuration. Prisms 56 and 356 have substantially planarfaces 216 and 416, respectively, which are made attachable to surface 10of window pane 4 with a good optical contact. An optical adhesive orencapsulant may be used to promote such optical contact.

A face 214 of prism 56 and a face 314 of prism 356 both areperpendicular to the plane of pane 4. A face 218 of prism 56 and a face418 of prism 356 are both sloped with respect to surface 10 so that theycan redirect light into pane 4 by means of TIR. A central reflectiveelement is provided between prisms 56 and 356. The reflective element isformed by symmetrically disposed, sloped reflectors 24 and 29 which aremirrored for good specular reflectivity.

Light source 30 represented by one or more light emitting diodes 32 isdisposed immediately adjacent to the opening between prisms 56 and 356.An optional heat sink 36 is provided on the back of each light emittingdiode 32 to promote natural cooling. Prisms 56 and 356 as well asreflectors 24 and 29 are configured work cooperatively to inject atleast a substantial portion of light emitted by LEDs 32 into pane 4 atangles allowing for TIR from surfaces 10 and 12 and towards therespective directions. Accordingly, prism 56 and reflector 24 should beconfigured to direct light towards direction 98 and prism 356 andreflector 29 should be configured to direct light towards an opposingdirection 298.

At the same time, the respective prisms and reflectors should beconfigured to minimize or prevent light re-entry onto light couplingoptical element 6. Accordingly, light emitted by source 230 and injectedinto pane 4 by element 56 will propagate in pane 4 by means of TIRsymmetrically away from the source. Light extracting features 20 (notshown) may be distributed along directions 98 and 298 at anypredetermined locations of pane 4. Such light emitting areas should beconfigured to extract the injected light towards a viewer or anypredetermined direction.

The transversal width of the area of optical contact of such symmetricaloptical element 6 with pane 4 may be defined based on reasoning similarto that used for obtaining Equation 3 above for an asymmetrical linearoptical element 6. Accordingly, in one embodiment, the maximumtransversal aperture A_(max) of the symmetrical element 6 may beselected based on the following relationship:

$\begin{matrix}{A_{\max} = {\frac{4d}{\sqrt{n_{I}^{2} - 1}}.}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

Thus, in an exemplary case of a glass window pane 4 (n_(I)≈1.51), thewidth of the symmetrical light coupling element 6 may be up to a factorof 3.5 greater than the thickness of the glass pane when the acceptanceangle is to be maximized.

Referring to FIG. 4, it is noted that the illustrated cross-section mayalso represent various non-linear configurations of optical element 6and may be applied to the case when optical element 6 has anaxisymmetrical configuration which can be obtained by rotating thecross-section of FIG. 4 around an axis.

The faces of prisms 56 and 356 and the walls of reflectors 24 and 29 arenot limited to having a linear profile in a cross-section and may havevarious concave or convex profiles. Particularly, reflectors 24 and 29may form a cuspated shape which may be well suited for enhancing theefficiency of light coupling into pane 4 or providing a specific lightdistribution pattern. Likewise, face 18 of FIG. 2 and FIG. 3 as well asfaces 218 and 418 of FIG. 4 are not limited to having a lineargeneratrix in a cross-section and may be also be formed by any curved orsegmented profile. The suitable profiles may be selected, for example,in response to the known angular distribution of light emitted by source30 or to the transversal size of light emitting diode 32 or itscollimating optics, if any. Alternatively or in addition to that, thesurface profiles may be selected based on the thickness of pane 4 andconfigured for reducing the light spillage into the environment. Any offaces 18, 218 or 418 may also be optionally mirrored to prevent lightescape at lower-than-TIR incidence angles.

FIG. 5 illustrates an embodiment of system 2 in which linear opticalelement 6 is represented by a band or strip of optically transmissiveprismatic film laminated onto surface 10 of window pane 4. The prismaticfilm has a smooth surface facing surface 10 and a linear pattern ofparallel microprisms facing outwardly from pane 4. The microprismaticpattern may be formed in the surface of the film by any suitable method,such as, for example, embossing, microreplication, and the like. Eachlinear microprism may have two planar faces 84 and 88 so that the prismarray is formed by a continuous sequence of alternating faces 84 and 88.

Linear light source 30 comprising an array of LEDs 32 is tilted withrespect to a normal to surface 10 in the illustrated cross-section so asto provide a skew incidence of the light beam into the light receivingface of window pane 4 and facilitate light injection at angles favorablefor TIR propagation. Light source 30 is also provided with a cylindricalcollimating lens 34 extending along the linear LED array. Thetransversal cross-sectional outline of lens 34 may be selected toprovide a narrow-angle collimation for the divergent beam emitted byLEDs 32. According to one embodiment, lens 34 may be selected fromcylindrical lenses providing the Full Width Half Maximum (FWHM) angle of30 degrees or less. According to one embodiment, FWHM may be less than20 degrees and, more preferably, less than 10 degrees.

By way of example and not limitation, a continuous strip TIR optic withpart numbers 10397 or 10398 commercially available from Carclo Opticsmay be employed for lens 34. Such or similar continuous strip optic maybe injection molded or extruded from PMMA (acrylics), polycarbonate, orany other suitable optically-clear plastic material.

According to an aspect of the present invention, referring to FIG. 5,the angles of each of the faces 84 and 88 with respect to the plane ofsurface 10 may be selected in a manner similar to selecting the anglesof faces 14 and 18 of FIG. 2. More particularly, each face 84 should beconfigured to direct light into the medium of window pane 4 at such anincidence angle with respect to a normal to surfaces 10 and 12 which isgreater than the TIR angle at said surfaces. Depending on the initialpropagation angle of a light ray, such function may involve somerefraction and bending the ray path accordingly.

It will be appreciated that, when the prismatic film forming linearoptical element 6 is index matched to the refractive medium of pane 4,light rays refracted by faces 84 will generally maintain about the samepropagation angles upon entering the pane. On the other hand, some raysmay be emitted by source 30 at sufficiently low out-of-plane angleswhich already make incidence angles with respect to a normal to surface10 greater than the TIR angle. Therefore, such rays may be injected intopane 4 without appreciable bending in which case the light couplingfunction of optical element 6 may be limited to suppressing refractionat surface 10 and thus preserving the TIR incidence angles for the raysbeing injected into pane 4.

Accordingly, in at least some embodiments, faces 84 do not have to benecessarily configured to refract the incident light and may bepositioned about perpendicularly to the incident beam to simply admitlight into the film material without appreciable refraction. Byapproximate matching the refractive index of the prismatic film materialto that of pane 4 and by providing a good optical contact of the filmwith surface 10, the refraction at surface 10 can be suppressed. In thiscase, light rays entering the prismatic about perpendicularly to faces84 will be injected into pane 4 without change in propagation direction.Thus, when the initial angle of the incidence rays with respect to anormal to surface 10 is greater than the TIR angle characterizing saidsurface and the ray maintains its direction upon entering the medium ofpane 4, the condition of TIR at both parallel surfaces 10 and 12 willautomatically be met and the injected light can further propagate inpane 4 in a waveguide mode.

Referring further to FIG. 5, the slope of each face 88 may be selectedto minimize the chance of light injection at incidence angles lower thanTIR with respect to a normal to surfaces 10 and 12. In at least someconfigurations of the prismatic film, this may be achieved by minimizingthe interaction of the face with the incident light beam emanated bysource 30. By way of example and not limitation, the apparent lightreceiving aperture of faces 88, as viewed from source 30, may beminimized by varying the slope of faces 88 with the distance from sourceand making each face 88 about parallel to the line of sight to source30. In one embodiment the slope of faces 8 with respect to surface 10decreases with the distance from light source 30. In one embodiment, theslope angle of farthermost faces 88, with respect to surface 10, may beless than 10 degrees or so. Furthermore, it may be preferred that atleast the farthermost faces 88 have even lower slope angles with respectto surface 10. This may have an added advantage of faces 88 acting asTIR reflectors in secondary interactions with light rays which have beeninjected into pane 4 by the preceding portions of the prismatic filmalong the propagation path. It will be appreciated that each face 88extending near parallel to surface 10 may still reflect a substantialpart of light rays already propagating in pane 4 in a waveguide mode asopposed to the case where face 88 would have a relatively high slopeangle with respect to surface 10.

FIG. 6 illustrates an exemplary arrangement of linear light emitting andlight injecting components of system 2 in an exploded perspective view.Window pane 4 of FIG. 6 exemplifies a framed glass window pane in abuilding facade or a glass pane of a storefront window.

Light-injecting optical element 6 is formed by a microstructuredprismatic film similar to that of FIG. 5. The prismatic film islaminated to a face window pane 4 in a bottom portion of the window. Theorientation of the prismatic film is such that its longitudinal axis 44is aligned parallel to the bottom edge of the window and the linearmicro-prisms of the film also extend generally parallel to the bottomedge. The face-mounted prismatic film is firmly attached to the glasssurface using a low-tack optically-clear adhesive so that there isprovided a good optical contact between the film and pane 4.

Light source 30 of FIG. 6 is formed by a strip of high-brightness LEDs32 interconnected with each other in a linear LED array and mounted to anarrow heat-conducting printed circuit board (PCB) 90. A metal channel136 is attached to the back of PCB 90 with a good thermal contact.Channel 136 may have a dual function. For example, it may provide astructurally rigid encasement for the LED array and also act as a heatsink for the LEDs 32.

Light source 30 is also associated with linear collimating lens 34extending parallel to the longitudinal axis of the LED array. Lens 34should also be properly positioned with respect to the LED strip so thatit can intercept all or at least a substantial part of the divergentlight beams emanated by individual LEDs and collimate said beams atleast in a plane perpendicular to the longitudinal axis of the LEDarray.

Collimating lens 34, light source 30 and channel 136 may be bundledtogether in a single linear light-emitting assembly. The light-emittingassembly may be further encased into a housing (not shown) with anopening for the emitted beam.

The light-emitting assembly of FIG. 6 is positioned slightly below theprismatic film attached to the face of pane 4, parallel to the ground.Additionally, as shown in FIG. 6, the light-emitting assembly is rotatedaround its longitudinal axis so that the beam emitted by source 30 andcollimated by lens 34 is generally pointed from bottom up, towards theprismatic film and generally towards the opposing (top) edge of pane 4,as indicated by direction 98. The light emitting assembly or any of itscomponents may be attached to the window's frame or directly to the faceof pane 4, for example, by using suction cups or adhesive. Other methodsof fixing the light emitting components of system 2 in front of opticalelement 6 may include attaching to the building's structural elementsadjacent to the window, such as walls, floor, ceiling, etc.

Depending on the configuration of the LED strip and on the thickness ofpane 4, the prismatic film may take the shape of a relatively narrowstrip or tape in which the linear prismatic features extend along thelongest film dimension. The width of the prismatic film may be selected,for example, based on Equation 3, and its length may approximate thelength of the LED strip. However, it is noted that other parameters ofsystems 2 components may also be taken into account, such as the beamspread of source 30, the size of individual LEDs 32, the transversalsize of the exit aperture of lens 34, and the like.

The prismatic film is configured to receive the collimated light beamemanated by the light-emitting assembly and admit such light beam intothe glass medium of pane 4 at angles permitting for TIR propagationwithin the pane. As explained in the above-discussed embodimentsreferring to FIG. 5, the interaction of the collimated beam withlight-injecting prismatic film may involve refraction and/or totalinternal reflection of rays by at least some prism facets and mayfurther involve the suppression of refraction at surface 10.

Depending on the optical clarity of the glass of pane 4, light maypropagate in a waveguide mode considerable distances along the verticalextent of the window panel. It will be appreciated that a fraction ofthe propagating beam may reach the vertical edges of pane 4 due to thebeam divergence in the respective plane. However, it will also beappreciated by those skilled in the art that the respective light rayswill generally have internal incidence angles onto the surfaces of thevertical edges greater than the TIR angle at these surfaces, as a resultof beam refraction by the light-receiving facets of the prismatic filmand as a result of pointing the light beam towards the top edge of thepane.

Therefore, unless the vertical edges of pane 4 are excessively rough orspecially sanded, which may cause significant light scattering anddecoupling from the waveguide mode, most of light rays reaching an edgeof the glass pane will undergo TIR from its surface and will thereforeremain in the pane. Accordingly, the vertical edges of the glass panelmay participate in the waveguiding function of pane 4 and may furtherpromote beam mixing, thus resulting a more homogeneous distribution oflight within the pane. It will further be appreciated that, in the caseillustrated in FIG. 6, the prevailing direction of light propagation 98in window pane 4 will generally point from the bottom edge to the topedge of the pane even when the beam divergence and reflections fromvertical edges of the pane are considered.

Provided that pane 4 has a sufficient optical clarity and low lightabsorption at least in the desired wavelengths, light extractingfeatures 20 (not shown in FIG. 6) may be positioned along direction 98at a considerable distance from the area where light is injected intothe pane. Since mixing divergent light beams from a series of LEDs 32 inpane 4 effectively distributes light through the volume of pane's bodyand since multiple reflections from opposing surfaces 10 and 12homogenize such light, a uniform illumination of light extractingfeatures 20 may be achieved even when such features are distributed overa relatively large area of the pane. Accordingly, decoupling light frompane 4 using evenly-lit light extracting features 20 may produce arelatively uniform glow from the respective portions of the pane.Considering that light extracting features 20 will deplete light frompane 4 along direction 98, such features may be configured to have avariable extraction rate depending on the distance from source 30. Forinstance, when light scattering elements are used for light extractionin features 20, the density of such scattering elements may be madeincreasing along direction 98.

Referring to both to FIG. 5 and FIG. 6, it is noted that this inventionis not limited to employing microsprismatic films for forming extremelylow-profile optical elements 6. At least in some embodiments, theprismatic film of FIG. 5 and FIG. 6 may be replaced, for example, by aholographic film having suitable light-turning and/or refractionsuppressing functions and being capable of injecting at least a portionof light entering its surface into pane 4 at angles favorable for TIR.In further examples and embodiments, optical element 6 may also includediffracting films, light scattering films or any other kind oflight-turning films which can be configured for light injection into awaveguide mode.

In one embodiment, optical element 6 may include a light transmittingfilm or a thin plate structure which has two or more layers and isconfigured to redirect light internally by embedded microstructurerather than externally by surface microstructures such as thoseillustrated in FIG. 5. In a non-limiting example, the embeddedmicrostructures may be formed by casting a liquid transparent polymerhaving a first refractive index over a microstructured surface ofanother polymer having a different second refractive index. It will beappreciated that such an overmolding or casting process will create acorrugated boundary between the different-index layers which replicatesthe shape of the surface microstructures of the second polymer. When theovermolded surface microstructures are represented by a parallel arrayof linear microprisms, the internal boundary will comprise an array ofalternating facets, each facet representing a refractive or TIR-reflective optical interface that can bend light that enters suchlayered optical structure.

It is noted that such embedded microstructures may be formed by anysuitable process that forms two or more layers of differentoptically-clear materials with a non-planar, preferably corrugatedboundary. Such microstructures may also be made using coextrusion of twoor more films having different refractive indices. Variousconfigurations of film-thickness optical structures that suitable foroptical element 6 may include the light redirecting films disclosed inco-pending U.S. patent application Ser. No. 13/662,311, incorporatedherein by reference in its entirety.

FIG. 7 illustrates an embodiment of face-lit waveguide system 2 in whichoptical element 6 is exemplified by a two-layer optically transmissiveplate having internal corrugated boundary with a prismatic (sawtooth)cross-sectional profile. It is noted, however, that the same type ofcorrugated boundary and essentially the same basic functionality mayalso be achieved using a film-thickness structure.

The plate of FIG. 7 is generally planar, has two opposing parallelsurfaces 76 and 16 and is formed by a first optically transmissive layer62 and a second optically transmissive layer 64. The plate is configuredfor a generally unimpeded light passage through its body where surface76 is configured as a light input surface and surface 16 is configured alight output surface.

Surface 16 is made optically smooth and attachable to the planar surface12 of window pane 4 with a good optical contact. Layer 28 of indexmatched adhesive or encapsulant is provided to promote such opticalcontact and adhesion.

Similarly to FIG. 6, light source 30 of FIG. 7 is exemplified by alinear array or strip of compact high-brightness LEDs 32. Thelongitudinal axis of the linear array extends parallel to thelongitudinal axis of prismatic corrugations of the internallayer-to-layer boundary of the plate. In a further similarity to FIG. 5,source 30 also comprises collimating TIR lens 34. Lens 34 has a linearextruded configuration and is designed to collect the divergent lightbeam emanated by LEDs 32 and collimate such beam to a narrower angle,directing it onto light input surface 76.

As illustrated in FIG. 7, source 30 and collimating lens 34 arepositioned to illuminate surface 76 at an angle. The transversalaperture of lens 34, its collimation angle and the tilt angle should besuch that source 30 can illuminate substantially the entire light inputsurface 76 of the plate. Additionally, the direction of light emissionshould be generally oriented towards the intended prevailing direction98 of light guiding in pane 4 and towards light extracting features 20(not shown).

Referring further to FIG. 7, layer 62 of optical element 6 has a firstrefractive index n₁ and layer 62 has a second refractive index n₂ whichis higher than n₁. It is preferred that there is at least a minimumrelative difference in refractive indices between layers 62 and 64 toenable effective light bending. The refractive indices n₂ and n₁ as wellas the angles of the prismatic facets of the boundary between the twolayers should be configured to enable injecting light ray emanated bysource 30 into window pane 4 at a relatively high angle with respect toa surface normal. Such angle should be greater than the TIR anglecharacterizing surface 10 and 12 and should provide for the subsequentlight propagation in a waveguide mode by bouncing from opposing surfaces10 and 12.

In a non-limiting example, the prismatic boundary facets which arefacing source 30 may be configured to intercept at least a substantialportion of light emission emitted light and perform the main lightbending function of element 2. As further illustrated in FIG. 7, suchfacets can be fairly steep, making the dihedral angles with respect tothe plane of pane 4 in the range from 85 to 90 degrees. The so highdihedral angles may be advantageously selected to most effectively bendlight rays within the body of element 6 towards the plane of window pane4 so that the minimum incidence angles needed for TIR at surface 10 canbe reached for at least a substantial part of the light beam.

In contrast, the boundary facets which are facing away from source 30may be configured to make substantially lower dihedral angles withrespect to such the plane of pane 4. Such dihedral angles may be furtherselected so that the respective facets are aligned along the prevailingpath of light propagation in the body of element 6. This may be useful,for example, for minimizing or eliminating the interaction of thenon-functional boundary facets with light and particularly forminimizing unwanted refractions and reflections from such facets whichmay result in light spillage.

It is noted that the internal boundary between layers 62 and 64 wasshown in FIG. 7 with just several prismatic features for theillustrative purpose only and the illustrated representation of theboundary and the dimensions of optical element 6 should not be viewed aslimiting this invention in any way. It should be understood that suchboundary may also be formed by a large array of micro-scale prismaticfacets which can be as small as a few micrometers across. Accordingly,the thickness of optical element of FIG. 7 may also be varied in a broadrange, from relatively thin and flexible films in the few-micrometerrange up to rigid plates or slabs of several millimeters thick.

FIG. 8 illustrates an embodiment of face-lit waveguide system 2 in whichlinear collimating element 6 has curvilinear surface profiles in atransversal cross-section and where it also combines the light-injectionand light-collimation functions. In the illustrated transversalcross-section, optical element 6 has a portion facing light source 30where it is shaped as a TIR collimating lens. Such optical element 6further features an opposing portion having planar face 16 attachable tothe planar surface 10 of pane 4. The curvilinear side walls 106 and 118of element 6 should preferably have smooth polished surfaces providingfor lossless TIR and should be configured to aid the light-collimatingand light-injection functions. Either one or both walls 106 and 118 mayalso have planar portions or can be formed by any suitable conjugate ofcurved and planar profiles.

The extent of planar face 16 along the prevailing propagation path ofwaveguide light should generally not exceed a predetermined length whichis defined by the maximum out-of-plane angle of the injected beam and bythe thickness of pane 4. Such length may be defined, for example byEquation 3 described above. Particularly, in an exemplary case of windowpane 4 made from glass, the extent of planar face 16 along direction 98should not exceed 1.8d, where d is the glass pane thickness.

System 2 may incorporate any masking elements to block light which mayescape from optical element 6 or from adjacent portions of pane 4 due toimperfect coupling. Some light may escape may occur, for example, due tosome rays striking surfaces 10 or 12 at incidence angles lower than thecritical angle of TIR. When applied to panel-type illuminated signage,masking the respective areas where the stray light may emerge from thesystem can provide an improved appearance and contrast of the images orpatterns visualized by features 20.

FIG. 9 depicts an embodiment of system 2 which includes a portion ofwindow pane 4, light source 30, light-coupling optical element 6,broad-area light extracting feature 20, a reflector 42, and lightmasking element 142.

Pane 4 of FIG. 9 is made from highly transparent glass material and hassufficiently smooth surfaces 10 and 12 to provide for lossless TIRpropagation along at least direction 98. Such pane 4 may be exemplifiedby a portion of glass storefront window in which surface 10 is facingthe interior of the store and opposing surface 12 is facing the outside.

Light source 30 includes a linear array of electrically interconnectedLEDs 32, structural channel 136 and heat sink 36 attached to channel136. Optical element 6 is exemplified by a strip of light transmittingfilm which is laminated onto surface 10 and configured to at leastpartially suppress the natural refraction at the air/glass interface.The light-coupling film may also be configured to bend at least raysthat enter surface 10 at sub-TIR angles and communicate greater-than-TIRangles to such rays.

Reflector 42 is positioned generally parallel to the prevailing plane ofpane 4 and is configured to reflect any uncoupled light rays that mayemerge from pane 4 along the extent of the light coupling film. Suchlight may emerge from pane 4, for example, as a result of decoupling ofcertain light rays by element 6 due to the secondary interactions withthe refraction-suppressing film. The decoupled rays may be recycled byreflecting from reflector 42 and entering pane 4 again which may furtherenhance light coupling efficiency. Thus, reflector 42 positioned atsurface 10 the opposite surface 12 of pane 4 may act as a light guidingchannel aiding the light coupling process.

Light masking element 142 is exemplified by an opaque film or thin sheetpositioned at the opposing face of pane 4. Such masking element may beuseful, for example, for blocking stray light and preventing the glarethat may result from some light leakage in the light coupling area.Element 142 may be configured to absorb, back-scatter or reflect lightemerging from surface 12.

The broad-area light extracting feature 20 is attached to surface 10 ofpane 4. It is made from an opaque or translucent back-scatteringmaterial and is configured to extract light from pane 4 and direct theextracted light out of the pane through the opposing surface 12.

In operation, window pane 4 acts as a face-lit waveguide deliverycomponent for light which is emitted by source 30 and injected into thepane by optical element 6. Pane 4 mixes and distributes the injectedlight so that at least a portion of it can be intercepted and extractedby light extracting feature 20.

By using the above example of pane 4 being a storefront window withsurface 10 facing inward, the extracted light can be emitted away fromthe store so that the glow of feature 20 may be viewed by an outsideobserver 660. The inside location of feature 20 may have advantages ofoffering protection from the outside environment or elements.Additionally, the inside location of most or all light emitting andlight management elements of system 2 may also have an added advantageof providing more convenience for the store personnel, easierserviceability and better protection from theft or vandalism.

Referring further to FIG. 9, system 2 may be implemented in analternative configuration where light extracting feature 20 is attachedto the outside face or surface of pane 4 and configured to haveforward-scattering properties. A yet further alternative may includeconfiguring light extracting feature 20 to provide the visibility of itsglow from both sides of pane 4.

FIG. 10 depicts an embodiment of system 2 which includes a framedportion of glass window pane 4, light source 30, light-coupling opticalelement 6, and broad-area light extracting feature 20. Window pane 4 hastwo opposing broad-area faces 100 and 120 which are generally smooth andplanar. An edge 700 of window pane 4 is covered by a frame member 360and is not easily accessible for light input. Therefore, in theillustrated case, the broad area faces of window pane 4 represents aconvenient alternative light input surface for injecting light into pane4. In the illustrated case, face 100 is chosen for light input.

Optical element 6 is formed by a rectangular planar plate or strip oflight transmitting material. Direction 98 indicates the intendedprevailing propagation path of light in window pane 4. Optical element 6is attached to face 100 with a good optical contact and aligned withrespect to pane 4 so that the longitudinal axis of the rectangularplanar strip extends perpendicular to direction 98 and parallel to face100.

Light source 30 included a plurality of LEDs 32 arranged in a lineararray extending parallel to the longitudinal axis of optical element 6.Each LED 32 is positioned in an immediate proximity to the light inputface 14 of element 6 and also in a close proximity to face 100 of pane4. The optical axis of each LED 32 is aligned about parallel to theplane of face 100 so that the light input face 14 of element 6 isilluminated from a perpendicular direction.

Light extracting element 20 is attached to face 100 in a differentlocation of pane 4 along direction 98. Such element 20 may includeretro-scattering film which carries an image print and is configured toemit light within a prescribed angular cone towards a perpendiculardirection when illuminated from the inside of pane 4. Thelight-scattering film of element 20 may be laminated onto face 100 withsufficient optical contact for providing light interaction with therespective light scattering features of the film.

In one embodiment, in order to maximize the efficiency of light inputinto pane 4 through its face 100, the thickness of element 6 and thetransversal size of source 30 may be selected to be considerably lessthan the thickness of the pane. In one embodiment, the transversal widthof the contact area of optical element 6 with face 100 may be selectedfrom Equation 3.

In operation, referring further to FIG. 10, the light beam emitted bysource 30 enters light coupling optical element 6 through face 14 andfurther propagates through the refractive material of element 6. It willbe appreciated by those skilled in the art that, although each LED 32may emit a highly divergent beam, the divergence angle will generally benarrower inside element 6 than in the air due to the difference inrefractive indices. The refractive index of glass and many opticallytransmissive plastics is around 1.5 or greater while the refractiveindex of air is around 1. At such difference in refractive indices,practically all rays in of the light beam striking face 14 will bendtoward the prevailing plane of element 16 sufficiently to enable furtherpropagation along direction 98 by means of TIR.

As illustrated in FIG. 10, some light rays entering element 6 throughface 14 can propagate directly into pane 4 via the coupling face 16. Theoptical contact between face 16 and face 100 suppresses refractiontowards a surface normal which would normally occur at the air/glassinterface of the bare surface of pane 4. As a result, such light raysmay enter pane 4 without appreciable changing the propagation directionand can thus maintain the greater-than-TIR incidence angle with respectto face 120. Once reflected from face 120, such rays will also reflectfrom the free portions of face 100 due to the parallelism of faces 100and 120.

As further illustrated in FIG. 10, some light rays entering element 6through face 14 will initially strike face 22 which is opposing face 16.Such rays will reflect from face 22 by means of TIR and willsubsequently enter window pane 4 via the coupling area formed by theoptical contact of face 16 and face 100. Likewise, due to the suppressedrefraction at the light coupling area, such light rays will continuepropagating within pane 4 in a waveguide mode.

Light rays propagating in a waveguide mode through window pane 4 mayeventually strike the optical interface formed by light extractingelement 20 and face 100. Light extracting element 20 decouples such raysfrom pane 4 and directs them towards viewer's eye 660. In oneembodiment, light extracting element 20 may be configured to providemaximum intensity of the extracted light along a surface normal 220.Depending on the application, element 20 may also be configured to emitlight from pane 4 in a broad angle of angles with respect to normal 220or emit a collimated beam towards the viewer. Light extracting elementmay also be configured to emit light into opposing directions thusproviding the viewability of its illuminated areas from both sides ofpane 4.

FIG. 11 shows a perspective schematic view of the embodiment illustratedin FIG. 10. Light source 30 is exemplifies by a side emitting flexibleLED strip having four individual LEDS 32. Each LED 32 each exemplifiedby a surface-mount LED device (SMD) which can be implemented in a fairlysmall package being less than 2 mm in height. The small size of SMD LEDSmakes them particularly suitable for coupling to face 14 of element 6.Such side-emitting flexible LED strips are available from varioussuppliers and may be based on a number of densely or sparsely packed SMDLEDs. For example, one type of side-emitting (also called side-view) LEDstrips is available from Elemental LED of Emeryville, Calif. (Stocknumber EL-IMGSDRIB12V). The side-emitting LED strip may be provided withan adhesive backing which can be used for attaching the strip to face100 of pane 4.

The light-transmitting rectangular plate or strip plate exemplifyingoptical element 6 may be attached directly to face 100 using an opticaladhesive, optically clear double-sides adhesive tape or curableencapsulant. Alternatively, an intermediate light coupling film or platemay be used as a layer between the plate and pane 4. In a furtheralternative, optical element 6 may be pressed against face 100 using anexternal fixture. The external fixture may be attached to frame 300 orto surface 100 of pane 4. A suction device such as a pair of suctioncups may also be used for attaching element 6 to face 100. When element6 is mounted to face 100 with some lasting pressure applied to it, aliquid or gel-type optical encapsulant may be used instead of a fullycured one, in which case element 6 may allow for its easierremovability.

Since forming illumination system 2 does not generally require surfacepenetration into the waveguide plate and either one or both of opticalelement 6 and light extracting feature 20 may be implemented in the formof an easily attachable and detachable films or decals, this may provideadditional utility of system 2 and lower its cost of ownership.Particularly, system 2 may be made easily deployable onto the existingwindows inside or outside of the residential, commercial or governmentalbuildings without the need of undesired changes or invasion to theexisting structures. Since at least some types of commercially availablefilms used for in-window signage are easily removable from smoothsurfaces, the film representing light extracting feature 20 may be madeinterchangeable without the need to replace the entire system. Thus, theilluminated pattern, its shape, color, size, and the like may be changedwith a relative ease and at low cost by merely removing the lightextracting film and laminating another one onto the same or any otherarea of pane 4.

FIG. 12 schematically illustrates an embodiment of system 2 whichemploys a plurality of elongated optical elements 50 and where eachelement 50 has alternative configuration and orientation with respect todirection 98 compared to element 6 illustrated in the preceding Figures.

Referring to FIG. 12, there is provided a planar waveguide 190 definedby opposing broad-area surfaces 410 and 412 and made from a highlytransmissive glass or plastic material. Light source 30 of FIG. 12 isexemplified by a side-emitting flexible LED strip similar to that ofFIG. 11. Each of LEDs 32 is provided with an individual light couplingoptical element 50 which has a form of a linear rod or bar which has arectangular transversal cross-section and which longitudinal axis 44extends parallel to the intended prevailing propagation direction 98 oflight in pane 4.

Each rectangular bar is made from an optically transmissive dielectricmaterial and has four longitudinal sides each preferably having anoptically smooth, polished surface. At least one terminal end of therectangular bar also preferably has a smooth surface 92 configured forlight input. One longitudinal side of each element 50 is disposed inoptical contact with surface 410 of waveguide 190 and is configured forlight output while the other three longitudinal sides are exposed to airand configured for reflecting light by means of TIR.

Such configuration and orientation of elements 50 allows each suchelement to receive light by its surface 92 and guide such light alongits longitudinal axis 44 by reflections from the three exposed sides inresponse to TIR and optical transmission. Each element 50 therefore actsa light guide or light pipe for at least some rays which are notinitially propagating towards the light output side. The light outputside of element 50 provides an outlet through which light can leak fromelement 50 into waveguide 190 along the propagation path.

An advantage of implementing optical element 50 in the form of a lightpipe may be that such configuration may allow for lower-loss light inputinto a thinner waveguide compared to, for example, the configurationillustrated in FIG. 11. The use of arrangement of optical element 6 andLED-based source 30 of FIG. 11 for injecting light into a waveguidewhich have a thickness considerable less than the thickness of the platerepresenting element 6 may be problematic. This problem arises from thefact that a sizeable fraction of light may escape from element 6 throughits face which is opposing the light input face 14. In contrast, theoptical element 50 of FIG. 11 may be configured to inject most lightinto the underlying waveguide even when the thickness of the waveguideis less than that of the respective rectangular rod. Since each LED 32emits a light beam which has some divergence in the plane of waveguide190, at least a portion of said beam will escape from the TIR envelopeof element 50 at each passage of the beam through the light output sideof element 50. The longer the longitudinal extent of the rectangular rodforming element 50, the more light will be injected into waveguide 190and the less light will remain within the TIR envelope.

The operation of light-guiding optical element 50 is further illustratedin FIG. 13 by example of light injection into waveguide 190 whichthickness is less than the transversal dimensions of the rectangularlight transmitting bar forming element 50.

Referring to FIG. 13, light coupling optical element 50 is shown in atransversal cross-section. A face 94 of element 50 represents the sideof the respective rectangular bar which is disposed in the opticalcontact with surface 410 of waveguide 190. A face 96 of element 50represents an opposing side of the bar. Faces 91 and 93 respectivelyrepresent the other two sides of the bar in the transversalcross-section.

A light ray 336 exemplifies light received by element 50 from anindividual LED 32 of light source 30. It will be appreciated that theoptical contact of face 94 with surface 410 provides for a generallyunimpeded light passage from optical element 50 into waveguide 190 andback. At the same time, it will be appreciated that the other threesides of element 50 as well as the portion of surface 412 of thewaveguide immediately below element 50 form a TIR envelope (indicated bya dashed box area 222 in FIG. 13). Such TIR envelope confines ray 336 sothat such ray propagates along the longitudinal axis of element 50undergoing TIR from said faces and from surface 412. Ray 336 continuespropagating trough the body of element 50 until it reached an openingbetween face 93 and surface 412. Ray 336 thus eventually escapes fromenvelope 222 and continues its propagation in the body of waveguide 190by means of TIR, which completes the injection of such ray into thewaveguide.

It will be appreciated that, when all rays emitted by the respective LED32 are considered, the prevailing direction of light propagation will begenerally parallel to the longitudinal axis of the rectangular rodforming optical element 50. While some rays may reach the opposite endof the rod and escape from system 2 without being injected intowaveguide 190, the percentage of such rays with respect to the entireemitted beam can be made relatively small.

There are several factors determining the amount of light which mayremain in element 6 uncoupled into waveguide 190. One such factor is therelation of the thickness of waveguide 190 to the cross-sectionaldimensions of optical element 50 as it determines the relative size ofthe gap in envelope 222 available for rays to escape. Another factor isthe relative length of element 50 with respect to the waveguidethickness and with respect to the transversal dimensions of the opticalelement. Generally, the longer the linear element 50 is the higherpercentage of light may escape from it. A yet another factor determiningthe escape rate from optical element 50 is the initial divergence oflight beam emitted by the respective LED 32. A greater divergence angle,especially in the plane of waveguide 190, will generally result in ahigher rate of light escape from optical element 50.

Referring to FIG. 12, the beam divergence should also be accounted forwhen selecting the spacing between adjacent pairs of LEDs 32 and opticalelements 50. Particularly, it may be preferred that such spacing besufficient to generally prevent or minimize interference between theadjacent LED-coupler pairs. In other words, light injected intowaveguide 190 through one optical element 50 should not enter intoanother optical element 50 since it may cause ray decoupling and loss ofillumination efficiency.

FIG. 14 provides a further illustration of the operation of rod-shapedoptical element 50. A cone 710 denotes the divergence of the light beamin the plane of surface 410 as it escapes from element 50 into theunderlying waveguide. LED 32 illuminating light input face 92 of element50 emits light generally towards direction 98. Obviously, even when suchlight further undergoes multiple reflections within optical element 50and enters into waveguide mode, the general propagation direction willbe preserved. Thus, light injected into waveguide 190 using theabove-described principles will continue propagating through the body ofwaveguide 190 along direction 98 and towards the predetermined locationof the waveguide.

FIG. 14 also illustrates an exemplary method of attaching opticalelement 50 to surface 410 of waveguide 190. Accordingly, a rectangularpiece of light transmitting film 780 is laminated onto surface 410 witha good optical contact. In a non-limiting example, the lamination may bedone using a low-tack adhesive or static cling properties of the film.Optical element 6 may be attached to film 780 using a relativelyhigh-tack adhesive. The adhesive may be applied in a liquid form or inthe form of a double-sided adhesive tape which should have sufficientoptical clarity.

In case of using a low-tack adhesive of static cling for attaching film780 to surface 410, the film may generally have a lower peel resistancethan optical element 50. Therefore, in one embodiment, the area of film780 can be made substantially larger than the area of the respectiveface 94 of optical element 50 which may providing a more reliableattachment of the film to the face of waveguide 190.

FIG. 15A through FIG. 15F illustrate various exemplary configurations oflinear optical elements 50 of FIG. 12. Particularly, FIG. 15A showsoptical element shaped in the form of a rod having a squarecross-section. In FIG. 15B, optical element 50 has the shape of arectangular bar. While light output face 94 of element 50 is shown tocorrespond to a longer side of the corresponding rectangle, it is notedthat any of the shorter sides may be designated for this.

FIG. 15C shows optical element 50 having the form of a half-round rodwith one terminal end designated as light input face 92 and the planarlongitudinal side designated as light output face 94. Optical element 50having a hexagonal rod configuration is shown in FIG. 15D. In FIG. 15E,optical element 50 is formed by a triangular optically transmissive. Byway of examples and not limitations, the respective transversalcross-section may have the shape of a right-angle isosceles triangle oran equilateral triangle. However, it should be understood that suchcross-section may be represented by any other type of triangle. In FIG.15F, optical element 50 is formed by a portion of a generally round rodwhere said rod has a planar longitudinal surface portion which isconfigured as light output face 94.

It is noted, however, that possible variations of the transversalcross-sectional shapes of linear optical element 50 are not limited tothe shapes illustrated in FIG. 15A through FIG. 15F or other precedingdrawings. Optical element 50 may have an elongated configuration whichtransversal cross-section may approximate other common two-dimensionalshapes including but not limited to pentagons, octagons, trapezoids,circles, circular segments or sectors, ovals, and the like.

FIG. 16 illustrates an embodiment of face-lit illumination system 2which employs rod-shaped optical element 50. The respective LED-basedlight sources 30 and elements 50 are grouped pairwise to createindividual light emitting/coupling structures. Each pair is positionedat the respective corner of framed window pane 4 and is configured toinject light into the pane. Each optical element is positioned so thatit directs light towards the respective direction 98 and within therespective angular cone 710 in the plane of pane 4. All directions 98converge towards the center of pane 4 so that the individual beamsinjected by source-coupler pairs are directed towards the central areaof the window where light extracting feature 20 is located.Image-carrying light extracting area 20 is configured to extract lightfrom pane 4 and direct the extracted light outwardly.

FIG. 17 depicts an embodiment in which light emitted by a single lightsource 30 is coupled into planar waveguide 190 though face 419 using abundle 776 of optical fibers 60.

Referring to FIG. 17, light source 30 can be any light emitting deviceproviding a beam of light that can be injected into a round aperture ofthe fiber bundle 776. Particularly, it may be is exemplified by a highbrightness LED coupled to a collimating or concentrating optic whichshapes the beam emitted by the LED according to the numeric aperture ofthe fibers 60 in bundle 776. By way of example and not limitation, theLED optic may be represented by a 15-mm, hex shaped concentrator lensmanufactured by Polymers Optics (Polymer Part #180) coupling into planarwaveguide 190. Such lens produces a 6 mm beam diameter at 14.5 mmdistance in front of the lens and is designed particularly for insertingLED beams into fiber bundles.

Fiber bundle 776 includes five optical fibers 60. In a non-limitingexample, optical fibers 60 may be about 2 mm diameter each. The entranceaperture of bundle 776 is located at a prescribed location from the LEDoptic (e.g., approximately 14.5 mm in Polymer Optics concentrator lensis used) to intercept at least a substantial part of light emitted bysource 30.

Fibers 60 are distributed along an edge 806 of waveguide 190 andattached to surface 410 by means of light transmitting film 780. Film780 is laminated onto surface 410 with a good optical contact andsufficient adhesion to resist peeling off while holding fibers 60.Fibers 60 are glued to film 780 in such a manner that there at least aportion of the surface of each fiber has sufficient optical contact withfilm 780 to enable light leakage into waveguide 190. In one embodiment,the ends of each fiber 60 may be provided with a planar surface portionsuch as, for example, is shown in FIG. 15F. In one embodiment, the endportion of each fiber may be flattened to facilitate bonding to film 780and optical contact.

Each fiber 60 is straightened in the light coupling area and alignedperpendicularly to edge 806 so that the fiber ends attached to film 780are generally parallel to each other and point towards the intendedprevailing light propagation direction 98.

The operation of the embodiment illustrated in FIG. 17 will be apparentin view of the above described embodiment of FIG. 12. Accordingly, lightemitted by source 30 and injected into fiber bundle 776 propagates ineach fiber 60 in a waveguide mode without appreciable leakage until thearea of film 780 is reached. The optical contact areas that each fiber60 forms with film 780 result in light escape from the fibers andinjection of such light into waveguide 190. Moreover, it will beappreciated that, despite the divergence of individual light beamsinjected by each fiber 60, the prevailing propagation direction 98 ofthe injected light in waveguide 190 will be generally parallel to thelongitudinal axes of the straight ends of the fibers. Accordingly,suitable light extracting features may be positioned along direction 98to extract light from a different location of waveguide 190.

The foregoing embodiments have been described upon the case where theplanar waveguide was exemplified by a highly transparent plate and awindow pane which has a relatively high optical clarity. However, thisinvention is not limited to this and may be applied to the case where arelatively low-clarity or even translucent panel or plate may be usedfor distributing and emitting light injected through the face of suchplate or panel. Many common objects may fall under this category, forexample partially-transmitting glass or plastic elements of buildinginterior or exterior, certain glass or plastic materials used forstorefronts or building facades may have intrinsic light-scatteringproperties, etched door windows or glass table tops, etc.

Alternatively, or in addition to that, various commercially availablesheets of optically transmissive materials may have surface texture thatscatters light propagating in the waveguide mode. Accordingly, althoughthe distance of light propagation is shortened with reducedtransparency, at least some of such plates, panels or sheets may act asface-lit planar waveguides extracting light along the entire propagationpath and provide a soft glow from the entire surface or at least itsportion adjacent to or surrounding the optical element 6. Thus,lower-clarity planar optical waveguides may still be employed for system2 in the manner proposed without the departure from the scope of thisinvention.

The structure and operating principles of the above describedembodiments of face-lit illumination systems may be applied withoutlimitations to any common glass or plastic objects which have theappropriate structure and sufficient optical clarity or transmissivityto act as planar waveguide. Examples include but are not limited toglass table tops, back-surface mirrors, glass or transparent-plasticdoors or inserts of furniture articles, screens, light transmittingsheets employed in large-area lighting panels, light diffusing sheets,and the like. For instance, one or more optical elements 6 or 92 may beattached to a face of a glass table top or a vanity mirror. Each of theoptical elements 6 or 92 may be provided with one or more light source30 in order to insert light into the respective glass slab or panel andforce its lateral propagation in a waveguide mode. A light extractingfilm may be attached to the face of such glass or panel in anotherlocation to extract light towards an observer. Either one or all of theattached optical components of system 2 may be made removable from theglass surface and may also be further made repositionable on the same ordifferent surface. Additionally, different types of light couplingoptical elements and light sources may be used within the same systemthus providing even greater degree of control over system configurationand light emission.

This invention is not limited in application to the planar light guidingpanels, sheetforms or slabs having strictly parallel broad surfaces, butcan also be applied to the case where the planar light guide has a wedgeconfiguration which may be exemplified by pane 4 being slightly taperedtowards one of the edges. A tapered configuration of the planarwaveguide may be advantageously selected, for example, for an improvedlight extraction where light can be injected by elements 6 or 50 intothe light guide along a direction pointing generally towards the taperededge.

Furthermore, system 2 may incorporate any number of auxiliary layersserving various purposes, such as, for example, providing additionalmechanical strength, environmental resistance, peel resistance, improvedvisual appearance, color, etc. Any optical interface between a layerformed by a lower refractive index transmissive medium and a layerformed by a higher refractive index transmissive medium may also beprovided with an intermediate optically transmissive layer, for example,for promoting the optical contact or adhesion between the layers. Theintermediate layer should preferably have a refractive index which isgreater than the lower of the two refractive indices at the givenoptical interface.

System 2 may further incorporate various color filters, inks, dyes orother devices or substances that change the color of the extractedlight. System 2 may also incorporate polarizing elements, fluorescentelements, fluorescent elements, light scattering or diffusing elementsand the like, which may be provided as separate layers or incorporatedinto the bulk material of the optical elements of source 30, the body ofoptical elements 6 or 50, pane 4, film 8 or its features 20.

Further details of operation of waveguide illumination system 2 shown inthe drawing figures as well as its possible variations will be apparentfrom the foregoing description of preferred embodiments. Although thedescription above contains many details, these should not be construedas limiting the scope of the invention but as merely providingillustrations of some of the presently preferred embodiments of thisinvention. Therefore, it will be appreciated that the scope of thepresent invention fully encompasses other embodiments which may becomeobvious to those skilled in the art, and that the scope of the presentinvention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more.” All structural, chemical, and functionalequivalents to the elements of the above-described preferred embodimentthat are known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe present claims. Moreover, it is not necessary for a device or methodto address each and every problem sought to be solved by the presentinvention, for it to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed under the provisions of 35U.S.C. 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for.”

What is claimed is:
 1. A light guide illumination system, comprising: aplanar light guide defined by two opposing broad-area surfaces andconfigured to guide light along the broad-area surfaces in response tooptical transmission and total internal reflection; a plurality ofdiscrete light sources arranged into a linear array and configured toilluminate the planar light guide; a plurality of highly elongated lightcoupling elements formed from an optically transmissive dielectricmaterial and configured for coupling light from the discrete lightsources to the light guide; and a plurality of light extraction featuresformed in at least one of the broad-area surfaces and configured forextracting light from the planar light guide; wherein the plurality ofhighly elongated light coupling elements is located between theplurality of discrete light sources and the plurality of lightextraction features; wherein each of the plurality of highly elongatedlight coupling elements longitudinally extends perpendicular to an edgeof the planar light guide; wherein each of the light coupling elementshas an optically transmissive light input face positioned perpendicularto a prevalent plane of the planar light guide and an opticallytransmissive TIR face positioned at an angle to the prevalent plane andconfigured to reflect light by total internal reflection; wherein theplurality of discrete light sources is positioned in an immediateproximity to the light input faces of the plurality of highly elongatedlight coupling elements; and wherein a transverse width of each of theplurality of highly elongated light coupling elements is approximatelyequal to or less than 2d√{square root over (n²−1)}, where d is thethickness of the planar light guide and n is the refractive index of thematerial of the planar light guide.
 2. An illumination system as recitedin claim 1, wherein each of the discrete light sources comprises alinear array of light emitting diodes.
 3. An illumination system asrecited in claim 1, wherein each of the discrete light sources has anelongated configuration and comprises multiple light emitting elementsarranged into a linear array.
 4. An illumination system as recited inclaim 1, wherein the plurality of discrete light sources comprisesside-emitting light emitting diodes attached to the planar light guide.5. An illumination system as recited in claim 1, wherein the pluralityof discrete light sources comprises a laser source.
 6. An illuminationsystem as recited in claim 1, wherein the plurality of light extractionfeatures includes a fluorescent material.
 7. An illumination system asrecited in claim 1, wherein the plurality of light extraction featuresincludes a material that converts light energy from a first wavelengthto a second wavelength, wherein the first wavelength is shorter than thesecond wavelength, and wherein the second wavelength is in the visiblewavelength range.
 8. An illumination system as recited in claim 1,wherein the plurality of light extraction features includes a materialthat converts light energy from a first wavelength to a secondwavelength, wherein the first wavelength is in a UV spectrum and thesecond wavelength is in a visible spectrum.
 9. An illumination system asrecited in claim 1, wherein the plurality of light extraction featuresis configured for changing the spectral distribution of lightpropagating in the planar light guide.
 10. An illumination system asrecited in claim 1, wherein the plurality of light extraction featurescomprises an image print made using inks that become conspicuous whenilluminated by a light source.
 11. An illumination system as recited inclaim 1, wherein each of the plurality of highly elongated lightcoupling elements includes a pair of opposing walls extendingperpendicular to the light input face and perpendicular to thebroad-area surfaces.
 12. An illumination system as recited in claim 1,wherein the planar light guide comprises a regular array of discretesurface relief features protruding from at least one of the broad-areasurfaces and separated from each other by regular spacing distances. 13.An illumination system as recited in claim 1, wherein each of theplurality of highly elongated light coupling elements comprises a linearright-angle prismatic structure or wedge formed in at least one of thebroad-area surfaces and having a taper generally pointing away from thelight input face.
 14. An illumination system as recited in claim 1,wherein each of the plurality of highly elongated light couplingelements comprises a prismatic surface structure having a transversalcross-section of a scalene right-angle triangle which hypotenuse isfacing the planar light guide.
 15. An illumination system as recited inclaim 1, wherein each of the plurality of discrete light sourcescomprises a light collimating element.
 16. An illumination system asrecited in claim 1, wherein at least one of the plurality of highlyelongated light coupling elements is optically coupled to the planarlight guide with refractive index matching.
 17. An illumination systemas recited in claim 1, wherein at least one of the plurality of highlyelongated light coupling elements is located at and aligned with an edgeof the planar light guide.
 18. An illumination system as recited inclaim 1, wherein at least one of the plurality of highly elongated lightcoupling elements is located at a distance from all edges of the planarlight guide.
 19. An illumination system as recited in claim 1, whereinat least a first portion of light emitted by the plurality of discretelight sources is communicated to the planar light guide withoutinteracting with the optically transmissive TIR face and at least asecond portion of light emitted by the plurality of discrete lightsources is directed to the planar light guide by means of reflectionfrom the optically transmissive TIR face.
 20. An illumination system asrecited in claim 1, wherein the plurality of highly elongated lightcoupling elements includes two identical linear prisms disposed in asymmetrical configuration.